Conventional liquid fuels synthesis (e.g. methanol synthesis) is conducted in gas phase, fixed-bed reactors and requires carefully controlled synthesis gas feed composition and flow. (See for example Roberts, G. W., et al, xe2x80x9cThe LPMEOH(trademark) Process: An Efficient Route to Methanol from Coal,xe2x80x9d Conference on Coal Gasification and Synthetic Fuels for Power Generation, San Francisco, Apr.14-18, 1985; see also Strelzoff, S., xe2x80x9cMethanol: Its Technology and Economics,xe2x80x9d Methanol Technology and Economics, Chemical Engineering Progress Symposium Series, No. 98, Vol. 66, AlChE, 1970.) This is because the liquid fuel synthesis reactions are generally highly exothermic, and the synthesis catalyst is generally very temperature sensitive. The catalyst loses its activity at high temperature. Furthermore, the equilibrium conversion of synthesis gas to a liquid fuel is higher at lower temperatures.
Therefore, the key consideration in designing a conventional liquid fuel synthesis reactor (also known as a slurry bubble column reactor or SBCR) is removal of the heat of reaction and maintaining operation in a specific temperature range, namely 428-518xc2x0 F. (220-270xc2x0 C.) for methanol. Isothermal operation at about 482xc2x0 F. is the optimum condition for methanol. Since synthesis gas feed composition and flow affect the reaction temperature, they are strongly controlled and normally held constant. Variations, especially rapid changes, are not well tolerated by the conventional gas phase, fixed-bed reactors. The heat of reaction is removed by either cold gas quenching (injection of cold synthesis gas along the length of the reactor) or production of steam (catalyst in tubes, boiling water on shell-side).
Gas phase methanol synthesis based on synthesis gas derived from gasification is commercially practiced. (See for example U.S. Pat. Nos. 4,888,1301; 5,266,281 and 5,284,878; see also Osterstock, E. R., et al, xe2x80x9cCoproduction of Methanol Adds IGCC Flexibility,xe2x80x9d Modern Power Systems, October 1997.) The synthesis gas feed composition and flow are carefully controlled per the considerations above. Normally, the synthesis gas feed composition is adjusted to be stoichiometrically xe2x80x9cbalancedxe2x80x9d (i.e. the ratio [H2 minus CO2]/[CO plus CO2] equals 2.0) or is H2-rich (i.e. this ratio is greater than 2.0).
In contrast to gas phase methanol synthesis, synthesis of methanol in a slurry bubble column reactor (SBCR) is a fundamentally different technology and more suitable for non-stoichiometric, variable synthesis gas feed composition and flow, including CO-rich synthesis gas feed. This is because the catalyst is slurried in an inert mineral oil that acts as a sink for heat removal. The large heat sink greatly eases temperature control, allowing essentially isothermal operation. Moreover, the liquid phase medium is highly robust and tolerant to wide-ranging and rapid changes in gas feed condition.
The objective of the present invention is to insure reliable product (especially a liquid fuel product but also including a gaseous product and/or a product that has a chemical use) production and limited purge gas flow when processing highly variable gas feed in a SBCR. Waste gasification art in general does not deal with tight control of downstream product and purge gas at fixed rates when the waste composition and flow vary over a wide range. xe2x80x9cWastexe2x80x9d here is meant to be heterogeneous, carbonaceous material that has low or negative value like municipal solid waste, refuse-derived fuel, industrial solid or liquid waste, sewage sludge, biomass waste, hazardous waste, toxic waste, refinery sludge or slop, plastic waste, automobile shredder waste, contaminated aqueous solutions, etc.
U.S. Pat. No. 5,788,723 (7) describes a process for the high temperature gasification of heterogeneous waste. The focus is on maintaining complete gasification in the face of varying heterogeneous feedstock character by controlling the oxygen lance operation. This patent does not address downstream needs.
U.S. Pat. No. 6,063,355 (8) describes a method and apparatus for treating wastes by gasification, and also the problem with changes in the quality of low-calorific value wastes when producing a downstream synthesis product, in this case ammonia. The invention solves the problem by adding a supplemental fuel having a high calorific value, like coal or coke, such that the mixed feedstock has a stable quality and quantity. This is an expensive solution, for ammonia or methanol production, and is totally different from the proposed concept.
U.S. Pat. No. 5,134,944 (9) describes, xe2x80x9cprocesses and means for waste resources utilization.xe2x80x9d Solid waste material is gasified and multiple products produced, including methanol, such that there are no remaining disposal problems. A key aspect of the invention is the use of a supplemental particulate carbon fuel like coal. The invention does not address the dynamics and needs of the downstream operation. This invention is totally different from the proposed concept.
International Patent Application WO 00/30973 (10) describes a gasification proves for making ammonia from heterogeneous waste feedstock. The invention does not attempt to maintain constant or optimum ammonia product flow and constant or limited purge gas flow.
The present invention is a control scheme for conversion of variable composition synthesis gas to a product (especially, but not limited to, a liquid fuels product such as methanol) in a three-phase or slurry bubble column reactor (SBCR). The control scheme allows one to achieve constant or optimum liquid fuel production and constant or limited purge gas flow with highly variable synthesis gas feed condition. This is accomplished by adjusting one or more of the following independent variables: recycle ratio, water addition, and bypass flow. The ability to achieve this control over a very wide range of synthesis gas feed condition is surprisingly strong given that only three variables are adjusted.